QUESTION TEXT: If a motor is sound-insulated, then it…
QUESTION TYPE: Must Be True
FACTS:
- Sound insulate (SI)–> Quiet for home (Q)
- Quiet for home (Q)–> Fine for institutions (I)
- Combined statement: SI –> Q –> I
Contrapositive:I–>Q–>SI - EM industries not quiet enough for home:
Q - Inference: EM industries not sound insulated:
SI
ANALYSIS: Usually, you should draw “must be true” questions, using letters. I kept the words in the first two statements so they’re clear to you. But, the combined diagram above is how I actually draw.
After you make a diagram, the next step is to see how the fact about EM industries fits into the logical chain. Since EM is Q, then they must be SI. You can and should be 100% certain about this kind of deduction on must be true questions, before checking the answers.
___________
- This is an incorrect reversal of the first diagram.
- CORRECT. This is true, according to the contrapositive diagram above.
- We don’t know. Being quiet enough for the home is a sufficient condition for being useful in institutional settings. EM fails to meet this sufficient condition. Failing to meet a sufficient condition doesn’t tell you anything. It’s possible some EM motors are quiet enough for institutions, or it’s possible that none are.
- This is an incorrect reversal of the first fact. Sound insulation guarantees quiet, but something could be quiet even if it’s not insulated.
- This is an incorrect negation of the second fact. Even if something is not quiet enough for use in the home, it might still be quiet enough for institutional settings.
Recap: The question begins with “If a motor is sound-insulated, then it”. It is a Must be True question. To practice more Must be True questions, have a look at the LSAT Questions by Type page.
Free Logical Reasoning lesson
Get a free sample of the Logical Reasoning Mastery Seminar. Learn tips for solving LR questions
Paul says
I am confused as to why E is an improper negation of the second fact. Wouldn’t an analogous example statement be:
“If the octane level of a gasoline is 89 or higher, then it is appropriate to use in Amigo brand car engines”
Then the negation would be:
“This gasoline has an octane level below 89, therefore it is not appropriate to use in Amigo brand car engines.”
It seems like there is a blurring/confusion of when something is necessary vs sufficient… i get that.. but the way I read fact two it reads like a necessary condition, not merely a sufficient one, because it’s a fact given in the context of technical regulations.
Is there a rule of thumb I can use moving forward to avoid this pitfall?
Or am I wrong more broadly here, in that my gasoline octane example above can’t be classified as a necessary condition statement either? Am I making an error by interpreting statements colloquially instead of interpreting the words as strictly literal?
TutorRosalie (LSATHacks) says
Let’s look at the stimulus again. We can have an overall diagram that looks like this:
if S -> Q -> I
-> ~EM
I added the relationship with EM on a second line since it only has a relationship with C and S, and not with I. I think here is where you went wrong. We’re told EM isn’t quiet enough. Let’s negate everything in this same format.
if EM -> ~Q -> ~S
We actually can’t infer anything about EM’s relationship with institutional settings since I is the necessary condition, and Q is the sufficient. Once it’s been negated, Q becomes the necessary condition, and you can’t infer leftward for the sufficient.
Chris says
Hey Graeme,
Thanks for doing these! As I practice for the December LSAT (in 2 days, ha), I find them immensely useful, albeit it is late in the game to help me much.
I have a question; why is A not correct? I see why B is correct, but I can’t see why A isn’t; if SI –> HA –> IS, then when something meets the necessary (of IS), it inherently means SI, hence sound-insulation is satisfied. Where and how am I wrong here?
Thank you so much for your time, and your most valuable input on r/Lsat!
FounderGraeme Blake says
You’re reversing the statement. It’s like saying: Cat –> Has tail
Scruffy the dog here has a tail, so he’s a cat. It’s bad logic. You can’t go from necessary to sufficient.